Editing Thermodynamics (section)

==System models==
[[File:system boundary.svg|class=skin-invert-image|200px|thumb|right|A diagram of a generic thermodynamic system]]
An important concept in thermodynamics is the [[thermodynamic system]], which is a precisely defined region of the universe under study. Everything in the universe except the system is called the [[Environment (systems)|''surroundings'']]. A system is separated from the remainder of the universe by a [[Boundary (thermodynamic)|''boundary'']] which may be a physical or notional, but serve to confine the system to a finite volume. Segments of the ''boundary'' are often described as ''walls''; they have respective defined 'permeabilities'. Transfers of energy as [[Work (thermodynamics)|work]], or as [[heat]], or of [[matter]], between the system and the surroundings, take place through the walls, according to their respective permeabilities.

Matter or energy that pass across the boundary so as to effect a change in the internal energy of the system need to be accounted for in the energy balance equation. The volume contained by the walls can be the region surrounding a single atom resonating energy, such as Max Planck defined in 1900; it can be a body of steam or air in a [[steam engine]], such as Sadi Carnot defined in 1824. The system could also be just one [[nuclide]] (i.e. a system of [[quark]]s) as hypothesized in [[quantum thermodynamics]]. When a looser viewpoint is adopted, and the requirement of thermodynamic equilibrium is dropped, the system can be the body of a [[tropical cyclone]], such as [[Kerry Emanuel]] theorized in 1986 in the field of [[atmospheric thermodynamics]], or the [[event horizon]] of a [[black hole thermodynamics|black hole]].

Boundaries are of four types: fixed, movable, real, and imaginary. For example, in an engine, a fixed boundary means the piston is locked at its position, within which a constant volume process might occur. If the piston is allowed to move that boundary is movable while the cylinder and cylinder head boundaries are fixed. For closed systems, boundaries are real while for open systems boundaries are often imaginary. In the case of a jet engine, a fixed imaginary boundary might be assumed at the intake of the engine, fixed boundaries along the surface of the case and a second fixed imaginary boundary across the exhaust nozzle.

Generally, thermodynamics distinguishes three classes of systems, defined in terms of what is allowed to cross their boundaries:
{{table of thermodynamic systems}}
As time passes in an isolated system, internal differences of pressures, densities, and temperatures tend to even out. A system in which all equalizing processes have gone to completion is said to be in a [[state (thermodynamic)|state]] of [[thermodynamic equilibrium]].

Once in thermodynamic equilibrium, a system's properties are, by definition, unchanging in time. Systems in equilibrium are much simpler and easier to understand than are systems which are not in equilibrium. Often, when analysing a dynamic thermodynamic process, the simplifying assumption is made that each intermediate state in the process is at equilibrium, producing thermodynamic processes which develop so slowly as to allow each intermediate step to be an equilibrium state and are said to be [[reversible process (thermodynamics)|reversible processes]].